ROEBLING MEDAL LECTURE: SUBDUCTION-ZONE METAMORPHISM—AN EVOLVING MINERALOGIC AND PLATE TECTONIC ADVENTURE

New continental crust forms/recycles today above subduction zones; Precambrian crust also may have been so produced. The rock record is clearest in Phanerozoic contractional orogens where subducted, disrupted, and resurrected lithospheric fragments contain zeolite-, prehnite-pumpellyite-, blueschist- and eclogite-facies metamorphic rocks. The latter are stable only under high- to ultrahigh-pressure conditions, as demonstrated over the past 50 years by thermodynamic calculations and experimental petrology. High P and low T are compatible with geologic occurrences and computed thermal structures of subduction zones.

Building on the research of many others, I synthesized what I thought was glaucophane at ~800° C and 1-2 kbar. The results were puzzling because mafic blueschist bulk compositions are similar to those of greenschists and amphibolites. With colleagues, we identified coexisting jadeitic pyroxene ± aragonite; these are neoblastic phases that clearly require temperatures of ~300° C at ~6-8 kbar. Evidently glaucophane schists form under high-P/low-T conditions. Geologic field work in California and Shikoku demonstrated that blueschists and eclogites form in penetratively deformed allochthonous slabs and nappes that verge seaward, requiring underflow of paleo-Pacific oceanic basement during metamorphism. Similarly in the blueschistic-eclogitic Alps, southward consumption of Tethys resulted in suturing of the European foreland against the structurally higher African plate. Astonishing discoveries of coesite and microdiamond inclusions in tough container minerals in the Western Alps, Western Gneiss Region, Norway, and Kokchetav massif, northern Kazakhstan showed that continental collision involved partial recovery of far more deeply subducted rocks than we had anticipated (30-40 kbar). But there's more! Garnet lherzolites from the central Alps, east-central China, and western Norway display mineral intergrowths and exsolution lamellae suggesting the former existence of majoritic garnet, requiring even greater depths of origin of host peridotites (70-150 kbar). Nano-minerals undoubtedly hold yet another key to unlocking the secrets of subduction. Mantle petrochemistry and dynamics control the evolving architecture of the crust, so the subduction process must be fully quantified for us to understand the nature of continents.